Generally, a double tubing bit type excavation tool, in which an excavation bit is mounted on the tip of a rod inserted into a cylindrical casing pipe, is frequently used for boring an excavation hole in a rock, etc. In this type of excavation tool, the rod transmits rotational force, thrust, and impact to the excavation bit to perform the excavation operation. In addition, a casing pipe is built into an excavation hole to prevent the excavation hole from collapsing (for example, refer to Patent Documents 1 and 2).
In detail, the excavation tool includes:    a casing shoe to which the thrust and impact are transmitted in the direction of an axis;    a casing pipe connected to the rear end of the casing shoe;    a rod, by which the thrust and the impact are transmitted in the direction of the axis together with the rotational force, arranged coaxially inside the casing pipe;    a device attached to a tip portion of the rod; and    an excavation bit, which is mounted on the tip of the device by restraining the rotation relative to the device.
The rotational force, thrust, and impact given by the rod make the excavation bit bore an excavation hole; and concurrently the casing pipe is built into the excavation hole.
In such an excavation tool, waste rocks produced during excavation must be discharged to the outside of the excavation hole.
For example, Patent Document 1 discloses an excavation bit in which an intake hole for discharging the waste rocks is open into the vicinity of the center of the tip face. The waste rocks are taken into the intake hole, and is discharged to the inner peripheral side of the casing pipe through the intake hole.
Additionally, Patent Document 2 discloses an excavation bit wherein                a face flute, which extends radially outward, is formed in the tip face; and        a flute portion, which is integrally connected to an outer peripheral end of the face flute and extends toward the axial rear end side, is formed in the outer peripheral surface of the excavation bit.        
In this excavation bit; the waste rocks are taken into the inner peripheral side of a casing pipe through the gap between the flute portion and the casing shoe, and is discharged to the outside of an excavation hole.    [Patent Document 1] Japanese Patent No. 3726179    [Patent Document 2] Japanese Patent No. 3903881